[llvm] [Docs][DirectX] Add relevant documentation of Root Signature (PR #149608)

[Justin Bogner via llvm-commits]

bogner wrote:

> Is there a way to view the rendered version? This change seems alright but I think seeing it rendered would help a lot

There's a "view file" button that takes you here: https://github.com/llvm/llvm-project/blob/f20706cae665c5e7283b37583088d148cfb22096/llvm/docs/DirectX/RootSignatures.rst
<img width="1190" height="593" alt="image" src="https://github.com/user-attachments/assets/9e8e5db6-db8d-4546-adde-606b8daf0f39" />

https://github.com/llvm/llvm-project/pull/149608